Optical transmission systems using coherent detection have mainly been deployed in long-haul networks in order to realize high speed transmission. However, coherent technology is now starting to be deployed in Metro networks in order to cope with the requirement of increasing transmission capacity. The main impediments for such applications have been the cost and footprint of the optical coherent transceivers, which usually consist of a tunable laser, an advanced format modulator, a coherent optical receiver, and a digital-signal processor (DSP). Optical integration techniques, such as the silicon-on-insulator technology (SOI), are widely used to reduce cost and minimize the size of an optical assembly in the form of a photonic integrated circuit (PIC) that comprises the optical modulator and the coherent optical receiver.
C. Doerr et al. “Single-Chip Silicon Photonics 100-Gb/s Coherent Transceiver”, Optical Fiber Communications Conference and Exhibition (OFC), 2014 describes a PIC integrating a full coherent receiver and a full advanced format modulator. The PIC contains all major optical functions needed for an optical coherent transceiver except the laser. The PIC comprises three fiber-optic interfaces (ports) for connecting two standard single-mode fibers and a polarization maintaining fiber. The PIC operates as follows: The optical power of a (tunable) continuous-wave (CW) laser enters one of the optical ports which is connected by an optical 1×2 power splitter to an input port of the advanced format modulator and a local oscillator input port of the coherent optical receiver. One half of the laser power is supplied to the coherent optical receiver and the other half to the optical modulator. A modulated optical transmit signal is output at another one of the three PIC ports that is connected to an output port of the integrated optical modulator. The optical signal to be received is supplied to the third PIC port which is connected to a receiving port of the integrated coherent optical receiver
As the CW laser is used for both the creation of an optical transmit signal, which uses the CW signal as an optical carrier that is modulated according to an (electrical) transmit signal, and as an optical local oscillator signal for the coherent detection of the optical signal to be received, this type of an optical transceiver PIC requires that the optical signal to be received has the same wavelength (in case of homodyne detection) or approximately the same wavelength (in case of intradyne detection) as the optical transmit signal. Such transceivers are usually deployed to realize bidirectional communication by using two separate optical paths (designated as dual-fiber working if the optical path is realized by one or more optical fibers) for the respective optical transmit signal and optical signal to be received. In this way, in-band distortions due to reflections (e.g. reflections at connector interfaces or reflections caused by Rayleigh scattering) are avoided. Of course, a single optical path may be used for transmitting the signals of both directions (designated as single-fiber working if the optical path is realized by one or more optical fibers) in order to save installation cost or cost for leasing the transmission path. However, additional optical components are required in this case in order to separate the transmission paths of the optical signals at both sides of the single optical path, e.g. optical circulators, or filters, which reduces the cost saving effect. Moreover, as already mentioned, in-band distortions limit the maximum transmission length of such a single-fiber working transmission system.
For this reason, optical transmission systems using single-fiber working, i.e. using a single optical path for the optical transmit signals in both directions, generally use differing wavelengths for the optical transmit signals. If conventional coherent optical transceivers are used, it is possible to connect separate lasers having differing wavelengths to the optical modulator and the coherent optical receiver, respectively.
If integrated optical transceiver modules, e.g. PICs as described above, which integrate an optical modulator and a coherent optical receiver in a single module that provides only a single port for connecting a CW laser, shall be used in order to realize an optical transmission that supports single-fiber working, two such integrated optical transceiver modules would be required. In one of the modules only the modulator would be used in order to create an optical transmit signal at a first wavelength and in the other module only the coherent optical receiver would be used for receiving the optical signal to be received that has a differing wavelength. Of course, such an application requires two CW lasers having correspondingly differing wavelengths. The remaining parts of the two optical transceiver modules would be unused.
It is further state of the art to realize a bidirectional optical transmission system using coherent detection that supports dual-fiber working by deploying two integrated optical transceiver modules as described above at each side of the optical paths. Two CW lasers having differing wavelengths are used at each side of the optical paths, wherein the two lasers at each side have the same or essentially the same wavelength. Each of the two optical CW signals is supplied to another one of the optical transceiver modules. In this way, two optical transmit signals are created at each side of the optical paths each having a different wavelength. Each of the dual optical transceiver devices comprises an optical path separating device, e.g. an arrayed waveguide grating, that is configured to route each of the two optical transmit signals to a selected one of the two optical paths. In this transmission system, one of the optical paths is used to transmit both of the optical transmit signals in one direction and the other one of the optical paths is used to transmit both of the optical transmit signals in the other direction. Each of the optical transmit signals to be received at the respective opposite side is supplied to the receiving port of the optical transceiver module which uses the CW signal having the same or essentially the same wavelength as a local oscillator signal in order to carry out the coherent detection.